Method of making magnetic transducers



y 1965 a. R. CURTIS ETAL 3,183,126

METHOD OF MAKING MAGNETIC TRANSDUCERS Filed April 19. 1960 United States Patent 3,183,126 METHOD OF MAKING MAGNETIC TRANSDUCERS Gerald R. Curtis, Duarte, James C. Kyle, Glendora, and

Glen Robinson, Pasadena, Calif assignors, by mesne assignments, to Physical Sciences Corporation, a corporation of California Filed Apr. 19, 1960, Ser. No. 23,267

7 Claims. (Cl. 148-12) This invention relates to magnetic transducers and to methods of making such transducers and, more particular- 1y, this invention relates to high temperature variable permeance transducers and to a method of making such transducers.

There are applications in which it is desirable to accurately measure a physical input such as pressure'in a high temperature environment. For example, in a nuclear reactor, measuring instruments and transducers are required for operation, in temperatue environments up to 1,000 degees Fahrenheit and in the presence of substantial radiation. In such environments, conventional instruments and transducers become inaccurate and often inoperable.

One conventional transducer utilized to transform a positional input to an electrical signal is a Variable Permeance Transducer. The input movement may be responsive, illustratively, to the action of a diaphragm of a pressure sensitive capsule. The Variable Permeance Transducer generally includes a first and a second Winding wound on a non-magnetic tube which encloses a moving element in the form of a core made of high permeability magnetic material. With the core centered in the tube, the permeances associated with the two windings are the same. As the core is moved toward either end, the permeance associated with one winding is increased, and it is decreased in the other winding. The result is that an output voltage is produced which is proportional to the displacement of the core.

Y The core is, of course, made of ferromagnetic materials and, therefore, is an alloy composed essentially of one or a combination of three elements, iron, cobalt and nickel which are the only three elements ferromagnetic at room temperature. The manufacture of a ferromagnetic alloy is generally a detailed process designed to enhance the desirable magnetic characteristics of the alloy. The magnetic properties of a single crystal of iron or nickel depend upon their crystallographic direction. With a proper crystal or grain orientation, certain magnetic characteristics are enhanced. Two methods are generally utilized to orient the crystals; cold rolling, and annealing, both in the presence of a magnetic field.

A core made of a conventional ferromagnetic alloy must be handled carefully because magnetic or thermal shocks and physical deformations degrade the magnetic properties of these special alloys. Even without shock, a variation of temperature changes the magnetic characteristics of the material. Conventional core materials such as Deltamax, Orthonal, 48 Alloy, Hypernik-V, Hy-Mu 80, 4-79 Mo-Permalloy and Super-.Permalloy (all US. trademarks) change as much as 10 to percent when heated even gradually to 1,000 degrees Fahrenheit. None of the conventional core materials are, accordingly, suitable for use to provide for an accurate magnetic transducer for use in high temperature environments. Moreover, some of these conventional core materials cannot withstand high temperatures. As a result, they oxidize at high temperatures and tend to disintegrate. The oxidation effectively separates the crystals of the ferromagnetic material. Further, thermal shocks or rapid changes in temperature speed the disintegration of the core materials.

3,133,126 Patented May 11, 1955 Moreover, conventional magnetic materials are all affected by the radioactivity present in nuclear reactors so that their magnetic characteristics change substantially responsive thereto. More specifically, with changes of temperature, and in the presence of radioactivity, the Curie point at which the material loses its ferromagnetic properties decreases, and the saturation flux density and rectangularity of the hysteresis loop changes. The coercive force also is changed. These changes are not readily predictable and, therefore, cannot readily be compensated for by components external to the transducer. Because of these various factors, suitable transducers for use in nuclear reactors have not heretofore been provided.

In a specific illustrative embodiment of this invention, a variable permeance transducer is provided including two matched impedances in the form of windings of a suitable material such as aluminum. Aluminum is eifectively transparent to radioactive particles. The aluminum is coated with a particular ceramic material which maintains its insulating properties at temperatures in the range of 1,000 degrees Fahrenheit and in the radioactive environment in the reactor. The inductive balance of the transducer is varied by means of the movement of a slug or core made essentially of a high temperature chromiumiron alloy. Features of this invention relate to a method of removing stresses from the core material and for magnetically stabilizing the core material so that neither the stresses nor the magnetic characteristics of the material effectively change with substantial variations of temperature. The stresses are removed and the stability of the material is improved by repeatedly thermoshocking the material to randomly orient the alloy crystals and to lock them in the random orientation. In one embodiment of the invention, a ferromagnetic stainless steel material is subjected to a number of different thermal shocks to stabilize the material both dimensionally and magnetically. The thermoshocks increase the modulus of elasticity and, hence, the hardness of the material. The permeability of the material is somewhat less than that of the conventional core materials but, due to the fact that the output voltage is derived from the magnetic unbalance associated with the two windings, the sensitivity of the transducer is maintained.

Other features of this invention pertain to the provision of means for compensating for any small changes of the magnetic characteristics of the core at the elevated temperatures. The compensating means is in the form of the resistivity of the aluminum windings which increases with temperature as does the permeability of the core. The change of magnetic characteristics of the core of this invention is quite small, illustratively 0.001 percent per degree Fahrenheit compared to 10 percent per; degree Fahrenheit for the conventional core materia s.

Further advantages and features of this invention will become apparent upon consideration of the following description when read in conjunction with the drawing wherein:

FIGURE 1 is a pictorial view of the variable permeance transducer of this invention;

FIGURE 2 is a sectional view of the variable permeance transducer of this invention;

FIGURE 3 is a pictorial view of the tube utilized in the variable permeance transducer of this invention; and

FIGURE 4 is an electrical representation of the variable permeance transducer of this invention.

Referring to FIGURES 1 through 4, the variable permeance transducer 10 is cylindrically shaped, and includes a tube or shell 11 made of a non-magnetic material. The material may illustratively be a non-magnetic steel. The tube 11 supports two windings 20 and 21 made of a material such as aluminum which is effectively a window for radioactive particles. Copper, which is conventionally utilized for transducer windings, becomes somewhat radioactive and changes its characteristics in the presence of the radiation. The aluminum wire of winding 20, coated with an insulating material, is Wound on the tube 11 between two bafiies 12 and 13 of the tube 11, and the aluminum winding 21, also coated with an insulating material, is wound on the tube 11 between two baffles 13 and 14. The bafiles 12, 13 and 14 are shown particularly in FIG- URE 3 together with two end baflles 19 and 16 positioned at the respective ends of the tube 11. The bafiles 12, 13 and 14- hold the windings 20 and 21 in place, and each includes a number of peripheral slots 17 for connecting leads between the windings'20 and 21 and for connections to external components, not shown. The windings 20 and 21 may be made of aluminum conductors mils in diameter and approximate gage of 30.

As illustrated in FIGURE 4, the two windings and 21 have three terminals 30, including a common terminal. The terminals are connected through circular openings 19 in the end baffle 15 of the tube 11. The terminals, or pins, 30 are supported by small ceramic insulator bushings, or beads, 34 in the openings 19 and may be made of non-magnetic stainless steel such as AISI 304. The tube 11 is coated with an inorganic ceramic material to fully insulate it from the windings 20 and 21. The ceramic material coating may be similar to the material of the ceramic bushings 34 and the insulator coating on the wires 20 and 21.

As indicated above, the aluminum windings 20 and 21 are also coated with a ceramic material which may be similar to that of the beads 34 and the coating of the tube 11. A suitable method for coating aluminum with a ceramic material which retains adequate insulating properties at temperatures in a range of 1,000 degrees Fahrenheit is disclosed in the copending patent application of John A. Earl, Serial No. 847,081, filed on October 19,

1959, now abandoned. As described in the copending patent application, the coating may consist of a mixture by weight of lead oxide from 70 to 76 percent silicon dioxide, from 10 to 14 percent bismuth trioxide, from 7 to 14 percent, and from 4 to 6 percent of any one of barium oxide, lanthium trioxide, magnesium oxide, calcium oxide and zinc oxide. The coating is substantially unaffected by nuclear flux because it has a low thermoneutron capture cross section of the order of only 30 barns. Moreover, as indicated above, the coating maintains its electrical insulation at temperatures on the order of 1,000 degrees Fahrenheit. The various ingredients of the mixture are thoroughly mixed and then smelted until homogenized at a temperature of approximately 2,100 degrees Fahrenheit. After being homogenized, the mixture is quenched in water and then ground through a fine mesh screen. The mixture then is coated on the aluminum and fired to a suitable firing temperature between 1,000 degrees Fahrenheit and 1,200 degrees Fahrenheit to cure the coating. The resistivity of the coating at room temperature is on the order of IX 10 ohms, and the resistivity at 1,000 degrees Fahrenheit is on the order of 4X10 ohms. As indicated above, the ceramic coating of the tube 11 and the ceramic beads 34 may be made of similar material. The windings 20 and 21 are enclosed by'a cylindrical tube 32 which, for example, maybe made of non-magnetic stainless steel having the AISI designation of 304. The tube 11 may be made of a similar material. The tube 32 may also be coated with the ceramic insulating material.

The tube 11 encloses a slug or core 25 which is made of magnetic material and which is movable longitudinallyin the tube 11. Two leads 26 and 27 are aifixed respectively to the core 25 and extend through the end bafiies 16 and 19 of the tube 11. The inductance presented by each of the two windings 20 and 21 and the coupling therebetween is determined by the longitudinal position of the core 25. With the core 25 centered, the inductance of the two windings 20 and 21 is identical so that equal signals developed across the windings are available at terminals 30.

As explained above, the magnetic characteristics of the core 25 must remain substantially constant throughout a temperature range up to 1,000 degrees Fahrenheit and in the presence of radiation. In order to achieve the required magnetic stability the various crystalline stresses within the core 25 are removed. The material is so stabilized as to have a variation of magnetic characteristics only 0.001 percent per degree Fahrenheit throughout a temperature range from minus 320 degrees Fahrenheit to 1,000 degrees Fahrenheit. Moreover, the Curie point of the material is increased and the. material is effectively immune to the radioactive environment.

These characteristics are achieved by subjecting a ferromagnetic high temperature material to a novel thermoshocking process. The material illustratively may be a magnetic stainless steel material or alloy consisting mainly of chromium and iron. One particular illustrative embodiment of the material includes by weight, 12 to 14 percent of chromium, 0.5 percent. nickel, 1.25 percent manganese, 1 percent silicon, 0.15 percent carbon and the rest iron. Such a stainless steel is conventionally designated AISI 416. The stainless steel is readily machinable and may also include a minimum of 0.07 percent by Weight of phosphorous, sulfur or selenium; or a maximum of 0.06 percent by weight of zirconium or molybdenum. Any one of the AISI 400 series of stainless steels may be magnetically stabilized. Stainless steel 416 is selected because it is also readily machinable. The AISI 300 types are unsuitable because they are not magnetic and cannot be heat treated. Before processing in accordance with this invention, the magnetic characteristics of the 400 series stainless steels are generally unsuitable for use in ambient temperature environments on the order of 1,000 degrees Fahrenheit.

One specific illustrative process for magnetically stabilizing the material to form a suitable core 25 for the variable permeance transducer 10 includes the following steps:

(1) The stainless steel material is heated to 1,550 de- I grees Fahrenheit and maintained at that elevated temperature for 8 to 10 hours;

(2) The heated material is then perature;

(3) The material is then immersed into liquid nitrogen at -320 to 325 degrees Fahrenheit and maintained in the liquid nitrogen for approximately 15 minutes;

(4) The cooled material is then warmed back to room temperature;

(5) The material is then heated to approximately 1,100 degrees Fahrenheit for 1 hour;

(6) The heated material is again returned to room temperature;

(7) The material is then reimmersed in the liquid nitrogen at approximately -320 degrees Fahrenheit for another 15 minute interval;

(8) The material is then returned to room temperature;

(9) The material is again heated to 1,100 degrees Fahrenheit for one hour;

(10) The material is returned to room temperature;

(11) Again, the material is thermally shocked to 320 degrees Fahrenheit in the liquid nitrogen for an interval of 15 minutes;

(12) The cooled material is then returned temperature;

(13) The substantially stabilized material is then cut and turned down to Ai-inch diameter to remove surface impurities and stressed areas generally at the periphery of the material;

(14) The cut material is then again heated to 1,100 degrees Fahrenheit for 1 hour to remove stresses in the material due to the machining operation;

(15) The heated material is returned to room temperature;

(16) The material is then thermally shocked to 320 cooled to room temto room degree Fahrenheit in the liquid nitrogen for 15 minutes;

(17) The material is then returned to room temperature;

(18) The piece of material is then turned down to 0.125 inch which is just slightly larger than that required for the particular magnetic core;

(19) A cylindrical hole 0.0509 inch in diameter is drilled through the core along its longitudinal axis;

(20) The drilled material is then heated up again to 1,100 degree Fahrenheit for 15 minutes, returned to room temperature, and dropped into the liquid nitrogen to remove the stresses in the material due to drilling;

(21) The material is then returned to room temperature;

(22) Finally, the piece of magnetic material is centerless ground to correct its external diameter to 0.116 inch $0.003 inch and to a length of 1.250 inches i0.0005 inch. The last step is essentially a surface cleaning operation.

The particular dimensions are, of course, merely illustrative and are given to illustrate the feature of machining the material to approximate size then thermoshocking the material again to remove machine stresses. Any of the conventional core materials would oxidize at the elevated temperatures and would fall apart during the repeated thermoshocks. for the core is a high temperature material which does not oxidize at the elevated temperatures. Moreover, the material is readily machinable so that only small stresses are introduced to the material by the machining operation.

The initial thermoshock cycles stabilize the temperatureexpansion characteristic of the core material. The machining operations are provided after this stabilization has been achieved in order to accurately provide the desired core dimensions.

A core 25 produced in accordance with this method is very stable and provides for minute changes of magnetic characteristics for temperatures up to 1,000 degrees Fahrenheit. Even the small change in magnetic characteristics, however, is compensated for by an opposite small change in the resistance of the aluminum wire with changes of temperature. The resistivity of the aluminum wire increases with changes of temperature to decrease the effective flux density develop in the core. Through the operating range, the resistivity illustratively increases from approximately 20 ohms per circular mil foot to approximately 60 ohms per circular mil foot. The Curie point of the core 25 is quite high, at approximately 1,580 degrees Fahrenheit.

Although this invention has been disclosed and illustrated with reference to particular applications, the prin ciples involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

1. A method of making a magnetic core having substantially constant magnetic characteristics over an extended temperature range, including the steps of, repeatedly thermoshocking a piece of ferromagnetic chromiumiron alloy of the A181 400 series between an elevated temperature of at least 1000 F. for at least one hour and a reduced temperature less than 200 F. for at least five (5) minutes to remove crystalline stresses in the alloy, then machining the piece of material to the required size :of the core, and then thermoshocking the machined piece between an elevated temperature of at least 1000 F. for at least one hour and a reduced tem perature less than 200 F. for at least five (5 minutes to remove stresses due to the machining.

2. A method of making a magnetic core having substantially constant magnetic characteristics over an extended temperature nange, including, the steps of heating and cooling in at least one cycle a piece of high tempera- The particular selected material ture term-magnetic material of the A181 400 series between temperatures in excess of 1000 F. for at least one hour and minus 200 F. for at least five (5 minutes; machining the piece to particular dimension; heating the machined piece to a temperature over 1000" F. for at least one hour; cooling the heated piece to a temperature less than minus 200 F. for at least five (5) minutes; and then heating the cooled piece to room temperature.

3. A method of making a magnetic core having effectively substantially constant magnetic characteristics over an extended temperature range, including, the steps of thermoshoclning a pieceof magnetic material of the A181 400 series in .a plurality of cycles by abruptly increasing the material to a temperature of at least 1000 F. for at least one hour; decreasing the temperature of the material gradually to approximately ambient room temperature; decreasing the temperature of the material to temperatures less than -200 F. for at least five (5) minutes, and increasing the temperature of the material gradually to approximately ambient room temperature to relieve crystalline stresses.

4. A method of making a magnetic core having substantially constant magnetic characteristics over an extended temperature range, including, the steps of cyclically and abruptly changing the temperature of a magnetic stainless steel piece of material of the A181 400 series between temperatures in excess of 1000 F. and less than -200 F. with the material being maintained at the extreme temperatures of the cyclical temperature changes for particular intervals of at least one hour at the high end of the temperature range and at least fifteen minutes at the low end of the temperature range, machining the piece of material to particular dimensions after a particular number of the cyclical temperature changes, and then abruptly changing the temperature of the machined piece of material between temperatures in excess of 1000 F. for at least one hour and less than 200 F. for at least five (5) minutes to remove stresses in the material due to the machining.

5. The method set forth in claim 4 in which the material is returned gradually to approximately room temperature after being subjected to one of the extreme temperatures and before being subjected to the Other of the extreme temperatures.

6. The method set forth in claim 5 in which the piece of material has a composition by weight of approximately 14 percent chromium, 0.5 percent nickel, 1.25 percent manganese, 1 percent silicon, 0.15 percent carbon and the remainder iron.

7. A method of making a magnetic core having substantially constant magnetic properties over an extended temperature range, including the steps of heating a memer of stainless steel of the AISI 400 series to a temperature over 1200 F. for an interval of at least one hour, returning the member of stainless steel gradually to approximately ambient room temperature, disposing the member of stainless steel in liquid nitrogen for at least five (5) minutes, returning the member of stainless steel gradually to approximately room temperature, reheating the memher of stainless steel to a temperature over 1000 F. for :an interval of .at least one hour, returning the member of stainless steel gradually to approximately ambient room temperature, disposing the member of stainless steel again in the liquid nitrogen for at least five (5) minutes, returning the member of stainless steel gradually to approximately ambient room temperature, machining and turning down the member of stainless Steel to particular dimensions thereby removing surface impurities and reducing stressed areas, reheating the machined member of stainless steel to a temperature over 1000 F. for at least one hour, returning the heated member of stainless steel gradually to approximately ambient room temperature, disposing the member of stainless steel in the liquid nitrogen for at least five minutes, and returning the cooled piece of stainless steel gradually to approximately ambient room temperature.

References Cited by the Examiner UNITED STATES PATENTS 2:5 43 FOREIGN PATENTS 643,367 9/50 Great Britain.

OTHER REFERENCES '5 preprint 10, article by Gippert et al. on Subzero Treatments, 1946 (10 pp).

Metal Progress, May 1949, pp. 643-648, article by Hoke et a1.

DAVID L. RECK, Primary Examiner.

01113 L. RADER, MARCUS U. LYONS, Examiners.

Dedication 3,183,126.-Gera?d R. Curtis, Duarte, James 0. Kyle, Glendora, zind Glen Robinson, Pasadena, Calif. METHOD OF MAKING MAGNETIC TRANSDUCERS. Patent dated May 11, 1965. Dedication filed June 3, 1970, by the assignee, Physical Sciences 001'p0mt2'0n. Hereby dedicates the entire term of said patent to the Public.

[Official Gazetze November 10, 1970.] 

1. A METHOD OF MAKING A MAGNETIC CORE HAVING SUBSTANTIALLY CONSTANT MAGNETIC CHARACTERISTICS OVER AN EXTENDED TEMPERATURE RANGE, INCLUDING THE STEPS OF, REPEATEDLY THERMOSHOCKING A PIECE OF FERROMAGNETIC CHROMIUMIRON ALLOY OF THE AISI 400 SERIES BETWEEN AN ELEVATED TEMPERATURE OF AT LEAST 1000*F. FOR A LEAST ONE HOUR AND A REDUCED TEMPERATURE LESS THAN -200*F. FOR AT LEAST FIVE (5) MINUTES TO REMOVE CRYSTALLINE STRESSES IN THE ALLOY, THEN MACHINING THE PIECE OF MATERIAL TO THE REQUIRED SIZE OF THE CORE, AND THEN THERMOSHOCKING THE MACHINED PIECED BETWEEN AN ELEVATED TEMPERATURE OF AT LEAST 1000*F. FOR AT LEAST ONE HOUR AND A REDUCED TEMPERATURE LESS THAN -200*F. FOR AT LEAST FIVE (5) MINUTES TO REMOVE STRESSES DUE TO THE MACHINING. 